1. Technical Field
The present invention relates to communications systems, and more particularly, to millimeter-wave transmission lines and hybrid couplers.
2. Discussion of the Related Art
A conventional balun is used to convert balanced or differential signals into unbalanced or single-ended signals. Baluns have found increasing use in circuits for millimeter-wave, radio frequency (RF), and high-speed wired applications. The integration of baluns with circuit elements has led to a reduction in power consumption, input/output ports, size and cost of balun-equipped circuits. Moreover, baluns for such circuit integration should be broadband and compact and have a low insertion loss and good return loss.
At low frequencies, for example, 1-5 GHz, integrated baluns are typically implemented using a spiral transformer. Spiral transformers work by exploiting magnetic coupling between inner wound coils of its spiral. The spiral transformer, however, is inherently narrow band due to its non-idealities. For example, the spiral transformer has a coupling factor of less than one, a finite self inductance on the primary and secondary coils and a parasitic capacitance. This leads to parasitics that have to be resonated out, thus limiting the operational bandwidth of the spiral transformer.
At millimeter-wave frequencies, a common way to realize the function of a balun is to use a “rat-race” or ring hybrid coupler. A ring hybrid coupler is typically implemented using three □/4 length transmission lines and one 3□/4 length transmission line all placed in a ring structure. The ring hybrid coupler has bandwidth limitations because the size of the ring structure is determined by the wavelength λ of the desired signal. The ring hybrid coupler does, however, provide common-mode and differential mode ports.
Recently, alternative topologies for baluns have been developed. One alternative topology for a balun is to use a transition from an unbalanced transmission line to a coupled or balanced transmission line. Examples of such back-to-back transitions are shown in FIGS. 1 and 2. As shown in FIG. 1, a balun 110 is connected to another balun 120 via a balanced stripline 130. The balun 110 consists of a portion of a microstrip (μS) transmission line 140, a transition 150 and a portion of the balanced stripline 130. The balun 120 consists of a portion of the balanced stripline 130, a transition 170 and a portion of another μS transmission line 160.
In another back-to-back transition shown in FIG. 2, a balun 205 is connected to a balun 210 via a coplanar stripline (CPS) 215. The balun 205 consists of a portion of a coplanar waveguide (CPW) 220, a transition 225 and a portion of the CPS 215. The balun 210 consists of a portion of the CPS 215, a transition 235 and a portion of another CPW 230. As further shown in FIG. 2, the CPW 220 includes a pair of ground traces 240a,b and a signal trace 245a, the CPS 215 includes a pair of signal traces 250a,b and the CPW 230 includes a pair of ground traces 240c,d and a signal trace 245b. 
In either back-to-back transition of FIGS. 1 and 2, the ground traces, for example, the ground traces 240a-d, are connected to the signal trace 250a using either a tapered 225, 235 or direct connection. These transitions 225, 235 have been shown to exhibit very broadband behavior. However, in such configurations, a direct current (DC) ground potential is imposed on the signal trace 250a. Thus, a differential circuit connected to the CPS 215 would include a DC blocking capacitor inserted between the circuit and the signal trace 250a and a dummy capacitor inserted on the signal trace 250b to balance the amplitude and phase shifts between the two signal traces 250a,b. The inclusion of these capacitors would, however, lead to discontinuities such as an impedance mismatch or insertion loss within the signal traces 250a,b. 